General Science eBook

[Illustration: FIGS. 54, 55.—­As the
pressure on the gas increases, its volume decreases.]

Pour more mercury into the open end of the tube, and
as the mercury rises higher and higher in the long
arm, note carefully the decrease in the volume of
the air in the short arm. Pour mercury into the
tube until the difference in level bd is just
equal to the barometric height, approximately 32 inches.
The pressure of the air in the closed end now supports
the pressure of one atmosphere, and in addition, a
column of mercury equal to another atmosphere.
If now the air column in the closed end is measured,
its volume will be only one half of its former volume.
By doubling the pressure we have reduced the volume
one half. Similarly, if the pressure is increased
threefold, the volume will be reduced to one third
of the original volume.

90. Heat due to Compression. We saw in Section
89 that whenever the pressure exerted upon a gas is
increased, the volume of the gas is decreased; and
that whenever the pressure upon a gas is decreased,
the volume of the gas is increased. If the pressure
is changed very slowly, the change in the temperature
of the gas is imperceptible; if, however, the pressure
is removed suddenly, the temperature falls rapidly,
or if the pressure is applied suddenly, the temperature
rises rapidly. When bicycle tires are being inflated,
the pump becomes hot because of the compression of
the air.

The amount of heat resulting from compression is surprisingly
large; for example, if a mass of gas at 0 deg.
C. is suddenly compressed to one half its original
volume, its temperature rises 87 deg. C.

91. Cooling by Expansion. If a gas expands
suddenly, its temperature falls; for example, if a
mass of gas at 87 deg. C. is allowed to expand
rapidly to twice its original volume, its temperature
falls to 0 deg. C. If the compressed air of a
bicycle tire is allowed to expand and a sensitive
thermometer is held in the path of the escaping air,
the thermometer will show a decided drop in temperature.

The low temperature obtained by the expansion of air
or other gases is utilized commercially on a large
scale. By means of powerful pistons air is compressed
to one third or one fourth its original volume, is
passed through a coil of pipe surrounded with cold
water, and is then allowed to escape into large refrigerating
vaults, which thereby have their temperatures noticeably
lowered, and can be used for the permanent storage
of meats, fruits, and other perishable material.
In summer, when the atmospheric temperature is high,
the storage and preservation of foods is of vital
importance to factories and cold storage houses, and
but for the low temperature obtainable by the expansion
of compressed gases, much of our food supply would
be lost to use.

92. Unexpected Transformations. If the pressure
on a gas is greatly increased, a sudden transformation
sometimes occurs and the gas becomes a liquid.
Then, if the pressure is reduced, a second transformation
occurs, and the liquid evaporates or returns to its
original form as a gas.